Add option to specify the database schema in addition to 'database' (e.g. for starrocks)

[LaurinBrechter]

I am currently running dbt, superset and apache starrocks (I know it's kinda niche) and want to also use dbt-superset-lineage for this. In starrocks, there is no such thing as a database, there's only a schema. Therefore, I have to specify my dbt models like this:

sources:
  - name: data
    schema: ...

specifying my models like this

sources:
  - name: data
    database: ...

would raise an error in dbt. dbt-superset-lineage doesn't work as of now as it only reads the database key in the yaml file and can't find the tables as database is None

In this PR I have added the option to specify a database or a schema.

• When both database and schema are given (like e.g. in postgres), only this combination will be pulled from.
• When either of the 2 are given we ignore the other (like in starrocks where schema would be given but database ignored)
• When none are given, we pull from all tables.

This also gives people more fine-grained control when they work with a DWH that has schema and database